WASHINGTON -- The composition of the U.S. family, particularly the black family, has undergone major changes over the past two decades, according to a new study by the U.S. Census Bureau.

Traditional families -- those headed by married couples -- have decreased from 87 percent of all family households in the nation in 1970 to 79 percent in March of this year, the study showed.

At the same time, the number of families maintained by women with no husband present in the house doubled over the last 20 years, from 5.5 million to 10.9 million this year.

But the figures on the number of black families maintained solely by women showed a more radical development, as those concerned with black-family structure have known for some time.

The trend is approaching the point at which such families will represent nearly half of all black families; now, they comprise 44 percent.

In addition, 6 percent of all black families are now maintained by men alone.

The result is that fully 61 percent of all black children now live in single-parent family households, the Census Bureau study showed.

Two areas spotlighted by the study, entitled "How We're Changing -- The Demographic State of the Nation: 1990," were Americans' incomes and the extent of poverty in the United States.

The median income of white families has risen slightly since 1970 -- from $32,713 for a total of 46.5 million families to $35,980 for 56.6 million families in 1989, the study found. The median income for black families has shown little change: $20,067 for a total of 4.9 million families in 1970, and $20,210 for 7.5 million families in 1989.

On a per capita basis, however, the study found that blacks have done better.

Their per capita income increased from $5,973 in 1970 to $8,750 in 1989, a rise of 46 percent. Whites' per capita income rose from $10,719 in 1970 to $14,900 in 1989, up 39 percent.

The study showed, nevertheless, that the goal of black-white parity appears to be a long way off.

Major civil rights and black affairs organizations, including the National Urban League and the Joint Center for Political and Economic Studies, have set a goal of black-white parity in income by the year 2000. But the study showed that blacks are not only nowhere near that stage, but are moving toward it at 0 0TC rate that would leave them considerably short of their goal.

Black families earned only about two-thirds -- 61 percent -- of the median income of white families in 1970, according to the study, and they have been unable to maintain even that low share.

Today, black-family median income represents only 56 percent of what a white family earns, according to the study.

Even on a per capita basis, the move toward parity is slow. Blacks earned 56 percent of whites' per capita income in 1970, the Census Bureau report showed. By 1989, blacks' per capita income had risen only to 59 percent of whites' earnings.

At the other end of the scale, the study's figures on black poverty showed a mixed bag.

The number of blacks below the poverty line has increased from 7.5 million in 1970 to 9.3 million in 1989.

But as the nation's black population has grown, the emergence of a black middle class has reduced the proportion of impoverished blacks in the total black population from 33.5 percent of 22 million in 1970 to 30.7 percent of 30 million in 1989.